A conventional planar linear pulse motor typically has a stator and an armature carrying mutually opposing magnetic pole teeth and is controlled so that some of the magnetic pole teeth of the stator may oppose the corresponding magnetic pole teeth of the armature. By appropriately energizing the drive coils provided in the magnetic pole teeth of the stator in a prescribed order, the opposing pairs of the magnetic pole teeth are made to shift in such a manner that the armature is moved by one fourth of the pitch of the magnetic pole teeth by each shifting of the energized state of the drive coils. To prevent inadvertent movement of the armature when the drive coils are not energized, a permanent magnet may be provided to produce a biasing magnetic flux in such a manner that a closed-loop magnetic flux may be produced across the magnetic pole teeth of the stator and the armature (refer to Japanese patent laid-open publication No. 62-64252). In other words, in such a linear pulse motor, when the drive coils are not energized, the armature is kept stationary and fixedly engaged by the force (which is called a cogging force) acting between the magnetic pole teeth of the stator and the armature which are magnetized by the biasing magnetic flux produced from the permanent magnet.
However, since this cogging force is not very strong, the armature could move and its stationary position could change when impacts, vibrations or other external forces were applied to the armature. Therefore, there was a possibility when such a planar linear pulse motor was used for the head drive of a floppy disk drive that its head would move abruptly, thereby causing not only faulty operation of the (read/write) head, but also damage to the linear pulse motor or to the head.
On the other hand, in order to more securely hold the armature, a large electric current must be supplied to some of the drive coils to increase their attractive forces, but this leads to an increase in power consumption.